The invention relates generally to spectroscopy, and particularly to a wavelength-modulation spectroscopy method and apparatus.
In recent years, wavelength-modulation spectroscopy has become the mainstream technology of laser-based gas analyzers. In wavelength-modulation spectroscopy, a laser is wavelength-modulated at a certain frequency, 1f, passes through a sample, and the transmission coefficient of the sample is demodulated by phase-sensitive detection at a certain harmonic of modulation frequency, to produce a harmonic spectrum. Predominantly, the second harmonic, 2f, is chosen for demodulation frequency. Phase-sensitive detection is an effective noise reduction technique and has made wavelength-modulation spectroscopy a highly sensitive method. The harmonic spectral signal magnitude reflects the concentration of an absorbing analyte in the sample. Specifically for 2f, the peak height of a 2f spectrum is linear with the analyte concentration within a certain range. Such linearity is the foundation of present 2f gas analyzes.
Wavelength-modulation spectroscopy traditionally is transmission-based, i.e., a harmonic spectrum is generated by demodulating the transmission coefficient of the sample. Because it is transmission-based, the harmonic spectral signal magnitude is inherently nonlinear with the analyte concentration, and can be considered linear with the analyte concentration only when the concentration is so low that absorbance is less than 0.05. Consequently, a gas analyzer utilizing wavelength-modulation spectroscopy has a narrow dynamic range, typically less than two decades (102). Beyond the dynamic range, the harmonic spectral signal magnitude is nonlinear with the analyte concentration. To compensate for such nonlinearity requires a calibration process that is laborious.
In addition, a tunable diode laser wavelength-modulation spectroscopy gas analyzer involves not only laser wavelength modulation but also laser intensity modulation, which asymmetrizes and complicates the resulting harmonic spectrum and makes it difficult to interpret. Therefore, there exists a need for a wavelength-modulation spectroscopy method and apparatus that can overcome these problems and disadvantages.